Like a bard would not play music and sing if you made Hod into a bard for example.
Bards are a different thing I think. Just like you can't turn a random NPC into a shopkeeper. If a steward, spouse or follower has blank dialogue or no dialogue at all, then it means that they do not have the voice files for it.
Try it yourself and see what happens. There is only one way to find out if your favorite NPC can be a steward, no?
I really don't know about stewards since I've never used them. Like I said, try it out and see if it works. Maybe it does.
Here's an easy way to make people as followers/spouses, and thus stewards if they are eligible by dialogue (again, some are and some aren't).
Create a .txt file in Data folder and give it a name of your liking.
paste these into it:
SetRelationShipRank Player 4
AddToFaction 0005A1A4 4
AddToFaction 0005C84D 4
SetPlayerTeamMate 1
Save and go to your game.
Enter console, click on target NPC.
Type bat [name of file]
For me it's bat Follower
Then talk to the NPC and see if you can take him/her as follower and spouse.
Yeah sure you can just use console but creating a bat file makes it faster. You need to type just one line instead of four.
And no need to create a folder. Data folder is already in your Skyrim folder, that's where the bulk of game data is.
You are thinking it the wrong way. Follower and spouse stuff do not care about the NPC, but the voice. Each NPC is assigned with a Voice Type, and each voice type represents a single voice actor. If a voice type has dialogue for any follower etc. functions, then all NPCs using the voice type will have that dialogue. Hod's voice type has follower dialogue, then so does Hod even if Hod himself wasn't intended as a follower.
Anyway, I gotta get to work. Try the commands I posted on NPC you'd like as spouse and follower and see what happens. It will most likely work.